Determining the coordination environment and electronic structure of polymer-encapsulated cobalt phthalocyanine under electrocatalytic CO2 reduction conditions using in situ X-Ray absorption spectroscopy

Photoelectrocatalytic CO2 Reduction to Methanol by Molecular Self-Assemblies Confined in Covalent Polymer Networks

Inspired by the porous structures of photosynthetic organelles, we report here a new type of photoelectrode based on a standalone macroporous conjugated polymer network (MCN) that converts sunlight into high-energy electrons for CO2 reduction to CH3OH. The MCN provides supramolecular cavities with sufficient functional groups that control the structures of photocatalytic assemblies, which circumvents the geometric limitations of traditional inorganic counterparts. Stabilized interfacial contact between MCN and photocatalysts is achieved by strong chemical linkages throughout the network. Solar irradiation of MCN with a cobalt-based catalyst generates highly reducing electrons for the reduction of CO2 to CH3OH at a conversion efficiency of 70%. Production of CH3OH sustains for at least 100 h, with a small decrease in yield rates. Scaling up the photoelectrode from 1 to 100 cm2 results in photocurrent generation stabilized at 0.25 A and continuous CH3OH production at a conversion efficiency of 85%, demonstrating the scalability and high performances.

Molecular Wiring of Electrocatalytic Nitrate reduction to Ammonia and Water Oxidation by Iron-Coordinated Macroporous Conductive Networks

Developing stable electrocatalysts with accessible isolated sites is desirable but highly challenging due to metal agglomeration and low surface stability of host materials. Here we report a general approach for synthesis of single-site Fe electrocatalysts by integrating a solvated Fe complex in conductive macroporous organic networks through redox-active coordination linkages. Electrochemical activation of the electrode exposes high-density coordinately unsaturated Fe sites for efficient adsorption and conversion of reaction substrates such as NO3 - and H2O. Using the electrode with isolated active Fe sites, electrocatalytic NO3 - reduction and H2O oxidation can be coupled in a single cell to produce NH3 and O2 at Faradaic efficiencies of 97 % and 100 %, respectively. The electrode exhibits excellent robustness in electrocatalysis for 200 hours with small decrease in catalytic efficiencies. Both the maximized Fe-site efficiency and the microscopic localization effect of the conductive organic matrix contribute to the high catalytic performances, which provides new understandings in tuning the efficiencies of metal catalysts for high-performance electrocatalytic cells.

Mitigating Cobalt Phthalocyanine Aggregation in Electrocatalyst Films through Codeposition with an Axially Coordinating Polymer

Cobalt phthalocyanine (CoPc) is a promising molecular catalyst for aqueous electroreduction of CO2, but its catalytic activity is limited by aggregation at high loadings. Codeposition of CoPc onto electrode surfaces with the coordinating polymer poly(4-vinylpyridine) (P4VP) mitigates aggregation in addition to providing other catalytic enhancements. Transmission and diffuse reflectance UV-vis measurements demonstrate that a combination of axial coordination and π-stacking effects from pyridyl moieties in P4VP serve to disperse cobalt phthalocyanine in deposition solutions and help prevent reaggregation in deposited films. Polymers lacking axial coordination, such as Nafion, are significantly less effective at cobalt phthalocyanine dispersion in both the deposition solution and in the deposited films. SEM images corroborate these findings through particle counts and morphological analysis. Electrochemical measurements show that CoPc codeposited with P4VPonto carbon electrode surfaces reduces CO2 with higher activity and selectivity compared to the catalyst codeposited with Nafion.

A replacement strategy for regulating local environment of single-atom Co-SxN4-x catalysts to facilitate CO2 electroreduction

The performances of single-atom catalysts are governed by their local coordination environments. Here, a thermal replacement strategy is developed for the synthesis of single-atom catalysts with precisely controlled and adjustable local coordination environments. A series of Co-SxN4-x (x = 0, 1, 2, 3) single-atom catalysts are successfully synthesized by thermally replacing coordinated N with S at elevated temperature, and a volcano relationship between coordinations and catalytic performances toward electrochemical CO2 reduction is observed. The Co-S1N3 catalyst has the balanced COOH*and CO* bindings, and thus locates at the apex of the volcano with the highest performance toward electrochemical CO2 reduction to CO, with the maximum CO Faradaic efficiency of 98 ± 1.8% and high turnover frequency of 4564 h-1 at an overpotential of 410 mV tested in H-cell with CO2-saturated 0.5 M KHCO3, surpassing most of the reported single-atom catalysts. This work provides a rational approach to control the local coordination environment of the single-atom catalysts, which is important for further fine-tuning the catalytic performance.

Active Sites of Cobalt Phthalocyanine in Electrocatalytic CO2 Reduction to Methanol

Many metal coordination compounds catalyze CO2 electroreduction to CO, but cobalt phthalocyanine hybridized with conductive carbon such as carbon nanotubes is currently the only one that can generate methanol. The underlying structure-reactivity correlation and reaction mechanism desperately demand elucidation. Here we report the first in situ X-ray absorption spectroscopy characterization, combined with ex situ spectroscopic and electrocatalytic measurements, to study CoPc-catalyzed CO2 reduction to methanol. Molecular dispersion of CoPc on CNT surfaces, as evidenced by the observed electronic interaction between the two, is crucial to fast electron transfer to the active sites and multi-electron CO2 reduction. CO, the key intermediate in the CO2 -to-methanol pathway, is found to be labile on the active site, which necessitates a high local concentration in the microenvironment to compete with CO2 for active sites and promote methanol production. A comparison of the electrocatalytic performance of structurally related porphyrins indicates that the bridging aza-N atoms of the Pc macrocycle are critical components of the CoPc active site that produces methanol. In situ X-ray absorption spectroscopy identifies the active site as Co(I) and supports an increasingly non-centrosymmetric Co coordination environment at negative applied potential, likely due to the formation of a Co-CO adduct during the catalysis.

Single-Atom Cobalt Catalysts Coupled with Peroxidase Biocatalysis for C-H Bond Oxidation

This paper reports a robust strategy to catalyze in situ C-H oxidation by combining cobalt (Co) single-atom catalysts (SACs) and horseradish peroxidase (HRP). Co SACs were synthesized using the complex of Co phthalocyanine with 3-propanol pyridine at the two axial positions as the Co source to tune the coordination environment of Co by the stepwise removal of axial pyridine moieties under thermal annealing. These structural features of Co sites, as confirmed by infrared and X-ray absorption spectroscopy, were strongly correlated to their reactivity. All Co catalysts synthesized below 300 °C were inactive due to the full coordination of Co sites in octahedral geometry. Increasing the calcination temperature led to an improvement in catalytic activity for reducing O2, although molecular Co species with square planar coordination obtained below 600 °C were less selective to reduce O2 to H2O2 through the two-electron pathway. Co SACs obtained at 800 °C showed superior activity in producing H2O2 with a selectivity of 82-85% in a broad potential range. In situ production of H2O2 was further coupled with HRP to drive the selective C-H bond oxidation in 2-naphthol. Our strategy provides new insights into the design of highly effective, stable SACs for selective C-H bond activation when coupled with natural enzymes.

Contemporary Strategies for Immobilizing Metallophthalocyanines for Electrochemical Transformations of Carbon Dioxide

Metallophthalocyanine (PcM) coordination complexes are well-known mediators of the electrochemical reduction of carbon dioxide (CO2). They have many properties that show promise for practical applications in the energy sector. Such properties include synthetic flexibility, a high stability, and good efficiencies for the reduction of CO2 to useful feedstocks, such as carbon monoxide (CO). One of the ongoing challenges that needs to be met is the incorporation of PcM into the heterogeneous materials that are used in a great many CO2-reduction devices. Much progress has been made in the last decade and there are now several promising approaches to incorporate PcM into a range of materials, from simple carbon-adsorbed preparations to extended polymer networks. These approaches all have important advantages and drawbacks. In addition, investigations have led to new proposals regarding CO2 reduction catalytic cycles and other operational features that are crucial to function. Here, we describe developments in the immobilization of PcM CO2 reduction catalysts in the last decade (2013 to 2023) and propose promising avenues and strategies for future research.

In-situ spectroscopic probe of the intrinsic structure feature of single-atom center in electrochemical CO/CO2 reduction to methanol

While exploring the process of CO/CO₂ electroreduction (CO_xRR) is of great significance to achieve carbon recycling, deciphering reaction mechanisms so as to further design catalytic systems able to overcome sluggish kinetics remains challenging. In this work, a model single-Co-atom catalyst with well-defined coordination structure is developed and employed as a platform to unravel the underlying reaction mechanism of CO_xRR. The as-prepared single-Co-atom catalyst exhibits a maximum methanol Faradaic efficiency as high as 65% at 30 mA/cm² in a membrane electrode assembly electrolyzer, while on the contrary, the reduction pathway of CO₂ to methanol is strongly decreased in CO₂RR. In-situ X-ray absorption and Fourier-transform infrared spectroscopies point to a different adsorption configuration of *CO intermediate in CORR as compared to that in CO₂RR, with a weaker stretching vibration of the C-O bond in the former case. Theoretical calculations further evidence the low energy barrier for the formation of a H-CoPc-CO^- species, which is a critical factor in promoting the electrochemical reduction of CO to methanol.

Catalyst Aggregation Matters for Immobilized Molecular CO2RR Electrocatalysts

Here, we detail how the catalytic behavior of immobilized molecular electrocatalysts for the CO₂ reduction reaction (CO₂RR) can be impacted by catalyst aggregation. Operando Raman spectroscopy was used to study the CO₂RR mediated by a layer of cobalt phthalocyanine (CoPc) immobilized on the cathode of an electrochemical flow reactor. We demonstrate that during electrolysis, the oxidation state of CoPc in the catalyst layer is dependent upon the degree of catalyst aggregation. Our data indicate that immobilized molecular catalysts must be dispersed on conductive supports to mitigate the formation of aggregates and produce meaningful performance data. We leveraged insights from this mechanistic study to engineer an improved CO-forming immobilized molecular catalyst─cobalt octaethoxyphthalocyanine (EtO₈-CoPc)─that exhibited high selectivity (FE_CO ≥ 95%), high partial current density (J_CO ≥ 300 mA/cm²), and high durability (ΔFE_CO < 0.1%/h at 150 mA/cm²) in a flow cell. This work demonstrates how to accurately identify CO₂RR active species of molecular catalysts using operando Raman spectroscopy and how to use this information to implement improved molecular electrocatalysts into flow cells. This work also shows that the active site of CoPc during CO₂RR catalysis in a flow cell is the metal center.

Multiple C-C bond formation upon electrocatalytic reduction of CO2 by an iron-based molecular macrocycle

Molecular macrocycles are very promising electrocatalysts for the reduction of carbon dioxide into value-added chemicals. Up to now, most of these catalysts produced only C₁ products. We report here that iron phthalocyanine, a commercially available molecule based on earth-abundant elements, can produce light hydrocarbons upon electrocatalytic reduction of CO₂ in aqueous conditions and neutral pH. Under applied electrochemical potential, C₁ to C₄ saturated and unsaturated products are evolved. Isotopic labelling experiments unambiguously show that these products stem from CO₂. Control experiments and in situ X-ray spectroscopic analysis show that the molecular catalyst remains intact during catalysis and is responsible for the reaction. On the basis of experiments with alternate substrates, a mechanism is proposed for the C-C bond formation step.

Metal single-site catalyst design for electrocatalytic production of hydrogen peroxide at industrial-relevant currents

Direct hydrogen peroxide (H₂O₂) electrosynthesis via the two-electron oxygen reduction reaction is a sustainable alternative to the traditional energy-intensive anthraquinone technology. However, high-performance and scalable electrocatalysts with industrial-relevant production rates remain to be challenging, partially due to insufficient atomic level understanding in catalyst design. Here we utilize theoretical approaches to identify transition-metal single-site catalysts for two-electron oxygen reduction using the *OOH binding energy as a descriptor. The theoretical predictions are then used as guidance to synthesize the desired cobalt single-site catalyst with a O-modified Co-(pyrrolic N)₄ configuration that can achieve industrial-relevant current densities up to 300 mA cm^-2 with 96-100% Faradaic efficiencies for H₂O₂ production at a record rate of 11,527 mmol h^-1 g_cat^-1. Here, we show the feasibility and versatility of metal single-site catalyst design using various commercial carbon and cobalt phthalocyanine as starting materials and the high applicability for H₂O₂ electrosynthesis in acidic, neutral and alkaline electrolytes.

Bioinspired and biomolecular catalysts for energy conversion and storage

Metalloenzymes are remarkable for facilitating challenging redox transformations with high efficiency and selectivity. In the area of alternative energy, scientists aim to capture these properties in bioinspired and engineered biomolecular catalysts for the efficient and fast production of fuels from low-energy feedstocks such as water and carbon dioxide. In this short review, efforts to mimic biological catalysts for proton reduction and carbon dioxide reduction are highlighted. Two important recurring themes are the importance of the microenvironment of the catalyst active site and the key role of proton delivery to the active site in achieving desired reactivity. Perspectives on ongoing and future challenges are also provided.

Considering the Influence of Polymer-Catalyst Interactions on the Chemical Microenvironment of Electrocatalysts for the CO2 Reduction Reaction

The electrochemical CO₂ reduction reaction (CO₂RR) is an attractive method for capturing intermittent renewable energy sources in chemical bonds, and converting waste CO₂ into value-added products with a goal of carbon neutrality. Our group has focused on developing polymer-encapsulated molecular catalysts, specifically cobalt phthalocyanine (CoPc), as active and selective electrocatalysts for the CO₂RR. When CoPc is adsorbed onto a carbon electrode and encapsulated in poly(4-vinylpyridine) (P4VP), its activity and reaction selectivity over the competitive hydrogen evolution reaction (HER) are enhanced by three synergistic effects: a primary axial coordination effect, a secondary reaction intermediate stabilization effect, and an outer-coordination proton transport effect. We have studied multiple aspects of this system using electrochemical, spectroscopic, and computational tools. Specifically, we have used X-ray absorption spectroscopy measurements to confirm that the pyridyl residues from the polymer are axially coordinated to the CoPc metal center, and we have shown that increasing the σ-donor ability of nitrogen-containing axial ligands results in increased activity for the CO₂RR. Using proton inventory studies, we showed that proton delivery in the CoPc-P4VP system is controlled via a proton relay through the polymer matrix. Additionally, we studied the effect of catalyst, polymer, and graphite powder loading on CO₂RR activity and determined best practices for incorporating carbon supports into catalyst-polymer composite films.In this Account, we describe these studies in detail, organizing our discussion by three types of microenvironmental interactions that affect the catalyst performance: ligand effects of the primary and secondary sphere, substrate transport of protons and CO₂, and charge transport from the electrode surface to the catalyst sites. Our work demonstrates that careful electroanalytical study and interpretation can be valuable in developing a robust and comprehensive understanding of catalyst performance. In addition to our work with polymer encapsulated CoPc, we provide examples of similar surface-adsorbed molecular and solid-state systems that benefit from interactions between active catalytic sites and a polymer system. We also compare the activity results from our systems to other results in the CoPc literature, and other examples of molecular CO₂RR catalysts on modified electrode surfaces. Finally, we speculate how the insights gained from studying CoPc could guide the field in designing other polymer-electrocatalyst systems. As CO₂RR technologies become commercially viable and expand into the space of flow cells and gas-diffusion electrodes, we propose that overall device efficiency may benefit from understanding and promoting synergistic polymer-encapsulation effects in the microenvironment of these catalyst systems.